The present invention relates to control of separately excited DC motors and, more particularly, to a control scheme for a separately excited DC motor wherein a field current signal is selectively boosted or de-boosted to expand the power envelope of the DC motor.
One problem with microprocessor control of separately excited motors results from the fact that the field flux expected from a given field current, as derived from the motor equations employed, is not always as expected. Specifically, actual field flux will differ from a given value, primarily as a result of magnetic hysteresis in the field poles. This variation in field flux is also attributable to motor-to-motor variations arising from manufacturing tolerances. Thus, while theory may predict optimal motor performance under given torque and speed conditions, in reality, the resulting operation is often not what is expected or desired. Accordingly, there is a need for a motor control scheme for a separately excited DC motor that accounts for the difference between the expected field flux and the actual field flux to improve the operation of the motor.
Another problem associated with microprocessor control of battery-powered separately excited motors arises when the voltage of the battery power source decreases with use. As the voltage decreases, the armature current decreases and the performance of the motor suffers. Accordingly, there is a need for a motor control scheme for a separately excited DC motor that additionally accounts for the decrease in battery voltage and armature current.